/Engine/Source/Runtime/HeadMountedDisplay/Public/IXRTrackingSystem.h
Device id 0 is reserved for an HMD. This should represent the HMD or the first HMD in case multiple HMDs are supported. Other devices can have arbitrary ids defined by each system. If a tracking system does not support tracking HMDs, device ID zero should be treated as invalid.
static const int32 HMDDeviceId = 0;